Shoppers on Oxford Street are being warned about a new scam, being used to con them out of their money.
It involves women - normally members of Romanian gangs - fooling people into handing over cash in exchange for worthless metal rings.
Westminster Council wardens say the scam has now become so common they are dealing with several incidents a day.
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Paramedics were on standby outside the police cordon as armed officers stood by the property.
A team of researchers has taken samples from the ten busiest London Underground lines to work out which are the dirtiest.
The action comes on top of an emergency timetable which has seen hundreds of trains a day being axed.